Generally, an electrecret microphone has a permanently charged dielectric (electrecret) between two parallel metal plates, which constitute the electrodes, one of the electrodes (i.e., the diaphragm electrode) being attached to a mobile diaphragm that can move in response to pressure variations of sound waves. The other electrode (i.e., the backplate) does not move. This diaphragm movement changes the distance, and therefore, the capacitance, between the diaphragm electrode and the backplate. Since the amount of charge in the electrecret remains constant, the voltage between the diaphragm electrode and the backplate changes in a manner which is inversely proportional to the change in capacitance, in accordance with Equation 1 shown below.Q=C·V  Equation 1
Usually this voltage is small (e.g., tens of miliVolt (mV)) and it is generated on a very high impedance, basically equal to the membrane capacitance. Therefore, there is a need to buffer and amplify this signal with a very high input impedance amplifier. Moreover, for economical reasons and compatibility with other microphone types, this amplifier has to be phantom powered, such that the direct current (DC) bias and the alternating current (AC) output signal share the same two physical wires. This characteristic is accomplished mainly by connecting the positive power supply line Vdd to the buffer amplifier through a load resistor RL and decoupling the output signal through a coupling capacitor CC, as illustrated in FIG. 1. Several implementations for such a buffer amplifier are widely known in the industry, as discussed below.
FIG. 2 is a schematic diagram of a conventional buffer amplifier circuit using a junction gate filed-effect transistor (JFET) device. Referring to FIG. 2, the amplification is provided by a JFET (as described in U.S. Pat. No. 5,097,224).
A major disadvantage of such an approach appears to be the large variability of the gain due to the electrical characteristics of industrial JFETs used in the circuit and the fact that the JFET is not a standard component in a digital metal oxide semiconductor (MOS) process.
Several other circuits have been developed to use feedback techniques in order to stabilize the gain versus process and temperature variations and to provide correct biasing, as illustrated in FIG. 3 and FIG. 4, as well as described in U.S. Pat. No. 5,239,579, U.S. Pat. No. 5,337,011, and U.S. Pat. No. 5,577,129.
Another implementation is further described in U.S. Pat. No. 6,160,450, and is illustrated in FIG. 5. FIG. 5 is a schematic diagram of a buffer amplifier circuit that requires very small and controllable input offset and a low value internal series resistor. One of the drawbacks of this circuit appears to be that it requires a very well controlled offset voltage at the input pair to set the general bias, as well as the fact that overall gain is determined by a direct ratio between the external load resistor R501 and an internal resistor R502, according to Equation 2.
                    gain        =                              R            501                                R            502                                              Equation        ⁢                                  ⁢        2            
Since the DC power of the amplifier/buffer is supplied through the load resistor R501 shown in FIG. 5, the value of this resistor cannot be made very large, otherwise a large DC drop will develop across it, requiring a very large DC voltage supply. Hence, in order to get reasonable gains according to Equation 2, the internal resistor R502 needs to be very small, in some cases well under 100 ohms, for example. The value of the resistor R502 and hence the gain is harder to control at low resistance values due to layout routing.
It would be advantageous, therefore, to provide a self-biased, phantom powered buffer amplifier device for an electrecret microphone without the aforementioned drawbacks.